Field of the Invention
The embodiments discussed herein are related to a technique for preventing deterioration in image quality due to rubbing between paper sheets with respect to a paper discharge device.
Description of the Related Art
With respect to a paper discharge device, various techniques have been developed for preventing misalignment of discharged paper sheets when paper sheets are stacked. When contact between a paper sheet and a discharge roller is released and the paper sheet is discharged in a not-in-contact state with stacked paper sheets, since the paper sheets fall freely and reach the stacked paper sheets, alignment of discharged paper sheets deteriorates. Therefore, a paper discharge device is proposed in which a discharge angle is made small so that the front end of a paper sheet to be discharged comes into contact with stacked paper sheets and then the paper sheet is placed on the paper sheet. FIGS. 18A-18C, and FIGS. 19A and 19B illustrate examples of such a discharge device.
FIG. 18A is a diagram illustrating how a paper sheet is discharged in a state in which there are no stacked paper sheets. Note that in regard to direction, in line with movement of the paper sheet 200 to be discharged, the rightward direction in the figure and the leftward direction in the figure are referred to as the upstream direction and the downstream direction, respectively.
By means of a discharge roller pair 104, the paper sheet 200 is discharged on a paper discharge tray 102 at a discharge angle such that the paper sheet 200 is directed downward to a downstream side in the S direction. The discharge angle is determined by an inclination of the discharge roller pair 104, etc. The front end of the paper sheet 200 to be discharged begins to come into contact with the paper discharge tray 102 while the paper sheet 200 is conveyed by means of the discharge roller pair 104.
As illustrated in FIG. 18B, when there are a few stacked paper sheets 210, the front end of the paper sheet 200 to be discharged begins to come into contact with the uppermost face of the stacked paper sheets 210 at almost the same position as that in FIG. 18A. The length between a contact start position and the front end position of the stacked paper sheets 210 is referred to as R, and R is the contact length over which the paper sheet 200 to be discharged contacts the uppermost face of the stacked paper sheets 210 until the paper sheet 200 to be discharged has been completely discharged.
As illustrated in FIG. 18C, as the stacked amount (the number of stacked sheets) of stacked paper sheets 210 increases, the contact start position of the front end of the paper sheet 200 to be discharged moves to an upstream side. Therefore, the contact length R becomes greater as compared with that when there are fewer stacked paper sheets 210, and the portion of the uppermost paper sheet of the stacked paper sheets 210 that is pushed out in the T direction increases by the paper sheet 200 to be discharged. The portion that is pushed out is not constant, which might deteriorate alignment of discharged paper sheets.
FIGS. 19A and 19B are diagrams illustrating how alignment of discharged paper sheets deteriorates when the size of the paper sheet 200 is larger. For example, FIG. 19A illustrates a case in which the paper sheet 200 is A4 size (210 mm×297 mm, as defined by ISO216), and FIG. 19B illustrates a case in which the paper sheet 200 is A3 size (297 mm×420 mm, defined by ISO216). When the paper sheet 200 is A3 size, the contact length R becomes greater and the contact time between the paper sheet 200 to be discharged and the uppermost face of the stacked paper sheets 210 becomes longer as compared with those when the paper sheet 200 is A4 size. Therefore, when the size of the paper sheet 200 becomes larger even though the stacked amount is the same, alignment of discharged paper sheets might further deteriorate.
In order to solve the problem that has been described in FIGS. 18A-18C, wherein the contact start position moves to the upstream side when the stacked amount increases, a paper discharge device is proposed which is configured to lower the paper discharge tray according to the stacked amount (For example, Japanese Laid-open Patent Publication No. H10-246998).
FIGS. 20A and 20B illustrate examples of such a paper discharge device. FIG. 20A illustrates a state in which the stacked amount of stacked sheets 210 is small, and FIG. 20B illustrates a state in which the stacked amount of stacked paper sheets 210 is large. As illustrated in FIG. 20B, a paper discharge device 150 detects an increase in the stacked height of stacked paper sheets 210 and lowers the paper discharge tray 102 by using a motor etc. by the increased amount. Thus, the contact length R may be made approximately constant regardless of the stacked amount of stacked paper sheets 210, and deterioration in alignment of discharged paper sheets due to the influence of the stacked amount may be prevented.